Silica glass article and manufacturing process therefor

ABSTRACT

A process of manufacturing a silica glass article comprising the steps of: (1) irradiating a silica glass article with electromagnetic waves to generate defects therein; and (2) immersing the thus irradiated silica glass article in an atmosphere comprising a hydrogen gas, thereby providing the resulting silica glass article with a characteristic that is effective for preventing it substantially from increasing its absorption within an ultraviolet region due to ultraviolet ray irradiation. Also disclosed are a silica glass article or a glass fiber produced according to the manufacturing process.

CROSS-REFERENCE

This application is a Continuation In Part of application Ser. No.09/080,247 filed May 18, 1998, now U.S. Pat. No. 5,983,673.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a silica glass article and amanufacturing process therefor. More particularly, the invention relatesto a silica glass article including an optical fiber suitable for use inan ultraviolet region, having an excellent initial transmissioncharacteristic and capable of preventing increase in a transmission lossoccurring due to ultraviolet ray irradiation, and to a manufacturingprocess therefor. The industrial utility of ultraviolet rays having awavelength of 160 nm to 300 nm has increased in industrial fields ofphotolithography, a laser process, sterilization, disinfection and thelike. A silica glass article according to the present invention is freefrom substantial deterioration caused by ultraviolet ray irradiation andcan be advantageously be used in such fields.

2. Description of Related Art

Silica glass articles have been used as light transmitting mediums, suchas optical fibers and various optical elements. In particular, use ofthe optical fibers having advantages of light weight, small diameter andno induction, has recently been widened in various industrial fieldsincluding communication, image transmission and energy transmission. Asone of the fields, use of the optical fiber to transmit ultraviolet rayshas been expected in the medical and precise processing fields. However,when glass is irradiated with ultraviolet rays, it deteriorates and itstransmission loss increases. That is, there arises a problem in thatdeterioration takes place because of ultraviolet ray irradiation. Sincethe transmission loss of a silica optical fiber having the silica glassas the core thereof is smaller than that of an optical fiber made ofmulticomponent type glass, the silica optical fiber is a preferredelement to transmit ultraviolet rays. However, the problem of thedeterioration which takes place because of ultraviolet ray irradiationremain unsolved.

It is possible for light transmission in silica glass to be superior tothat in air if the wavelength is not longer than 200 nm. The reason forthis is that dissociation of oxygen gas takes place because ofultraviolet ray irradiation in air. Therefore, a high transmission canbe expected if the deterioration which takes place because ofultraviolet ray irradiation can be reduced in the wavelength region notlonger than 200 nm.

It has been considered that the deterioration which takes place becauseof ultraviolet ray irradiation is mainly attributed to a defect inglass. In the present invention, the “defect in glass” means a brokenportion of the glass network structure or a portion of the glass networkstructure that is stretched due to a distortion of the glass and is aptto break easily. FIG. 4 shows a plurality of examples of reporteddefects in glass of silica glass. As representative defects in glass,defects related to E′ center (≡Si•) and oxygen-deficient type defects(≡Si—Si≡) are exemplified. The above-mentioned defects in glass absorbultraviolet rays at wavelengths of 163 nm, 215 nm and 245 nm. It hasbeen considered that the foregoing defects in glass occur in a glasssynthesized in an atmosphere somewhat lacking in oxygen, or in a glasshaving a low concentration of OH groups.

As a technique for reducing deterioration due to ultraviolet rayirradiation of silica glass, a technique has been disclosed inJP-A-5-147966 (hereinafter referred to as Document (1)) (The term “JP-A”used herein means an unexamined published Japanese patent application),in which the content of OH groups in a pure silica core is adjusted tofrom 10 ppm to 1000 ppm, the contents of F (fluorine) is adjusted tofrom 50 ppm to 5000 ppm and the contents of Cl (chlorine) is adjusted tosubstantially zero. An optical fiber thus obtained has an excellentinitial characteristic of transmitting ultraviolet rays and is capableof reducing deterioration due to ultraviolet ray irradiation becausefluorine is contained in a specific amount.

There are several known techniques that are not aimed to improvedeterioration due to ultraviolet ray irradiation but are related to animprovement in radiation resistance of a fiber for transmitting visiblerays or near infrared rays. For example, JP-A-60-90853 (hereinafterreferred to as “Document (2)), has suggested a process in which any oneof a glass soot body, a transparent glass preform and an optical fiberis processed in a hydrogen atmosphere to delete defects in the glass soas to improve the radiation resistance of the optical fiber. In theforegoing document, only a result of measurement of an increase in theloss experienced with respect to a near infrared rays having awavelength of 1.3 μm is described. In addition, the effect of improvingthe ultraviolet ray resistance obtained by the above-mentioned processdisappears within about two months.

In “Improvement in Radiation Resistance of Optical Fiber by HydrogenTreatment and γ-Ray Irradiation”, Tomon, Nagasawa, et al. pp. 1-213,Vol. 1, papers for lectures in National Conference of Semiconductor andIts Material Section of Electronic Communication Society, 1985, issuedin 1985 by Electronic Communication Society (hereinafter referred to as“Document (3)”), a process has been reported for the purpose ofpreventing increase in light absorption of a pure silica-core opticalfiber at a wavelength of 630 nm (visible ray) occurring due to γ-rayirradiation. In this document, two-step treatment for an optical fiberis performed. In the first step, an optical fiber is doped with hydrogenmolecules and then, in the second step, is irradiated with γ-rays. Thus,seeds (precursors) of defects in the glass are converted into defectsthat absorb photon energy of a 2 eV band. Then, hydrogen previouslydispersed in the fiber in the previous step and the defects in glass arechemically bonded to each other so as to improve the radiationresistance in the visible ray region. Also in the Document (3), there isno description about the characteristic of the fiber against ultravioletrays.

U.S. Pat. No. 5,574,820 (hereinafter referred to as “Document (4)”),suggests an optical fiber and its manufacturing process that serves as ameans for preventing increase in a loss in a visible ray region when apure silica core fiber is used as an image fiber for transmittingvisible rays in a radiation field. The proposed optical fiber ismanufactured by previously irradiating pure silica core fiber withradiation in a large dose of 10⁵ Gy or greater, so that increase in theloss in a visible ray region having a wavelength of from 400 nm to 700nm does not exceed 30 dB/km. Moreover, a process for manufacturing theoptical fiber has been suggested, but the characteristic in theultraviolet ray region has not been described.

JP-A-5-288942 (hereinafter referred to as “Document (5)), as in Document(4), has suggested a process for improving radiation resistance of animage fiber for transmitting visible rays. In the process, an imagefiber is irradiated with g-rays in a large dose of 10⁷ Roentgen to 10⁹Roentgen (10⁵ Gy to 10⁷ Gy) and then is heated in a hydrogen atmosphere.Also no description about the characteristic in the ultraviolet rayregion has been made in the above-mentioned document.

In the Document (2), hydrogen is added so that the radioactiveresistance of the optical fiber in the near infrared rays is improved.Recently there have been disclosed several processes in which hydrogenmolecules are added to silica glass in an attempt to improve ultravioletray resistance. For example, JP-A-3-23236 (hereinafter referred to as“Document (6)”) suggests silica glass in which OH groups are containedin an amount of 100 ppm or higher, substantially no oxygen defect existsand hydrogen gas is contained, so that ultraviolet ray resistance isimproved. JP-A-5-32432 (hereinafter referred to as “Document (7)”)suggests a process, in which deterioration due to ultraviolet rayirradiation is prevented by controlling the concentration of hydrogen insilica glass to 1.5×10¹⁷ molecules/cm³ or higher. Moreover, theconcentration of chlorine is made to be 100 ppm or lower to reducehydrogen consumption in glass when ultraviolet ray irradiation isperformed so as to maintain ultraviolet ray resistance. JP-A-6-16449(hereinafter referred to as “Document (8)”) suggests silica glass whichhas improved ultraviolet ray resistance by designing to contain OH groupin an amount of 100 ppm or lower and chlorine in an amount of 200 ppm orlower, and to have a hydrogen concentration of 10¹⁶ molecules/cm³ orlower, a refractive index fluctuation of 5×10⁻⁶ or lower and abirefringence of 5 nm/cm or lower. U.S. Pat. No. 5,668,067 (hereinafterreferred to as “Document (9)”) suggests silica glass in which the amountof OH groups is 50 ppm or smaller and hydrogen is contained by at least10¹⁸ molecules/cm³ and which is free from optical damage if the silicaglass is irradiated with 10⁷ pulses of KrF laser, the output of which is350 mJ/cm². U.S. Pat. No. 5,679,125 (hereinafter referred to “Document(10)”) suggests silica glass which has improved ultraviolet rayresistance because hydrogen molecules are added to silica glass to whichfluorine has been added.

JP-A-7-300325 (hereinafter referred to as “Document (11)”) suggests aprocess which is able to improve ultraviolet ray resistance by meanssimilar to that suggested in Document (5) in such a manner thathydrogen-molecule-contained silica glass is irradiated with γ-rays so asto make the concentration of hydrogen in the irradiated silica glass tobe 5×10¹⁶ molecules/cm³ or higher so that the ultraviolet ray resistanceis improved. JP-A-9-124337 (hereinafter referred to as “Document (12)”)suggests a process with which the ultraviolet ray resistance is improvedby irradiating glass containing hydrogen molecules at a concentration offrom 2×10¹⁷ molecules/cm³ to 5×10¹⁹ molecules/cm³ with ultraviolet raysof 150 nm to 300 nm for 20 hours or longer.

The Document (1) discloses an optical fiber having an excellent initialtransmission characteristic of ultraviolet rays. However, a satisfactoryeffect cannot be obtained to prevent the deterioration due toultraviolet ray irradiation. On the contrary, absorption caused at theabsorption edge of ultraviolet rays is enlarged undesirably. Therefore,adjustment of an optimum amount of addition cannot easily be achieved.

No description has been made about the deterioration due to ultravioletray irradiation in each of the Documents (2) to (5) relating toimprovement in the radiation resistance required for transmitting nearinfrared rays. As described later, the processes adapted to a fiber fortransmitting visible rays or a fiber for transmitting near infrared rayscannot maintain the effect to prevent deterioration due to ultravioletray irradiation for a required period of time. Moreover, unsuitablemeans for an optical fiber for transmitting ultraviolet rays have beenemployed.

The processes disclosed in the Documents (6) to (11) are arranged insuch a manner that the contents of OH groups, F or Cl are adjusted.Although the above-mentioned adjustment of the components attains aneffect of initial defects in glass, a satisfactory effect cannot beattained to reduce defects induced by ultraviolet rays.

The hydrogen treatment employed in the processes disclosed in Documents(6) to (12) is such that the defects in glass caused by ultraviolet rayirradiation and the hydrogen molecules dispersed in the glass by thehydrogen treatment are bonded to each other so that increase inabsorption of light is restrained. The restraining period, however, islimited to a period of time in which hydrogen molecules remain in theglass. Since the processes disclosed in the Documents (6) to (12) aremainly adapted to a bulk-form glass member, the volume of the glassmember is sufficiently large with respect to the velocity at whichhydrogen in the glass is dispersed. It is considered that hydrogenmolecules remain in the member for a long time and thus ultraviolet rayresistance can be maintained.

If the techniques in the Documents (6) to (12) are adapted to an opticalfiber, hydrogen is undesirably dispersed out the outside in a shorttime. Thus, there arises a problem in that the ultraviolet rayresistance cannot be maintained. That is, hydrogen molecules in anoptical fiber (having an outer diameter of 125 mm) are generallygradually discharged to the outside of the optical fiber at a roomtemperature and the concentration is lowered to about 1/10000 in abouttwo months as shown in FIG. 6. That is, the above-mentioned restrainingeffect is effective in only about two months after the hydrogentreatment has been performed. Therefore, increase in the absorptioncannot be restrained for a long time with the conventional techniques.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a silica glass article for use as an optical fiber or a bundlefiber for transmitting ultraviolet rays, having an excellenttransmission characteristic of ultraviolet rays and exhibiting asatisfactory resistance against deterioration due to ultraviolet rayirradiation.

Another object of the present invention is to provide a silica glassarticle substantially free from considerable deterioration even ifirradiated with ultraviolet rays having a wavelength not longer than 200nm and having superior light transmission as compared to that in air.

Another object of the present invention is to provide a silica glassfiber having a sufficient resistance against deterioration due toultraviolet ray irradiation and to provide a method of producing thesame without damaging its coating.

Still another object of the present invention is to provide a silicaglass article having excellent resistance against deterioration due toultraviolet ray irradiation.

A still further object of the present invention is to provide amanufacturing process for the foregoing silica glass articles.

Other objects and effects of the present invention will become apparentfrom the following description.

The above described objects of the present invention have been achievedby providing a process of manufacturing a silica glass articlecomprising the steps of:

(1) irradiating a silica glass article with electromagnetic waves togenerate defects therein; and

(2) immersing the thus irradiated silica glass article in an atmospherecomprising a hydrogen gas, thereby providing the resulting silica glassarticle with a characteristic that is effective for preventing itsubstantially from increasing its absorption within an ultravioletregion due to ultraviolet ray irradiation.

The electromagnetic waves can be ultraviolet rays, vacuum ultravioletrays, X rays or γ rays having a photon energy of not less than 3.5 eVwith which defects in glass can be generated.

An advantageous dose of the electromagnetic waves for irradiation isfrom 10 Gy to 10⁴ Gy.

Step (2) is advantageously carried out when the partial pressure of thehydrogen gas is from 0.5 atm to 10 atm and the temperature is not lowerthan room temperature.

The process can include the additional step of (3) irradiating again thesilica glass article that has been subjected to the step (2), withelectromagnetic waves while hydrogen molecules remain therein.

It is advantageous for the silica glass article to contain hydrogenmolecules at a concentration of not lower than 1×10¹⁶ molecules/cm³ atthe beginning of the step (3).

After the step (3), an additional step (4) can be carried out forreducing the hydrogen molecules that are remaining in the silica glassarticle to not higher than 1×10¹⁶ molecules/cm³.

The above-described process of manufacturing a silica glass article isadvantageously applied to manufacturing an optical fiber. Step (2) isadvantageously carried out at a partial pressure of the hydrogen gas offrom 0.5 atm to 10 atm and at a temperature in a range from roomtemperature to a highest temperature with which coating on the opticalfiber is not damaged. Step (2) should be carried out at a temperature offrom 80° C. to 200° C.

The optical fiber can be a bundle fiber formed by bundling a pluralityof optical fibers or an optical fiber for a bundle fiber before wound.

According to another aspect of the invention, there is provided a silicaglass article which is manufactured by a process comprising the stepsof:

(1) irradiating a silica glass article with electromagnetic waves togenerate defects therein; and

(2) immersing the thus irradiated silica glass article in an atmospherecomprising a hydrogen gas, thereby providing the resulting silica glassarticle with a characteristic that is effective for preventing itsubstantially from increasing its absorption within an ultravioletregion due to ultraviolet ray irradiation;

The process can include a further step of (3) irradiating again thesilica glass article that has been subjected to the step (2), withelectromagnetic waves while hydrogen molecules remain therein.

The process can further include after the step (3), a step of (4)reducing the hydrogen molecules that are remaining in the silica glassarticle to not higher than 1×10¹⁶ molecules/cm³ by allowing to stand inthe atmosphere or by heating at 80° C. or lower.

This invention also provides an optical fiber comprising a core and aclad having a refractive index lower than that of the core, the opticalfiber having the characteristic that when 10⁸ pulses of KrF excimerlaser having a wavelength of 248 nm are applied to the optical fiber, asultraviolet ray irradiation for an evaluation of ultraviolet resistance,at an output of 10 mJ/cm² and a pulse frequency of 100 Hz, the opticalfiber as measured with a sample of one meter length has a transmittanceof not less than 90% of the transmittance measured prior to theirradiation in a wavelength region of ultraviolet ray from 160 nm to 300nm.

The process can further include a step of (3) irradiating again theoptical fiber that has been subjected to the step (2), withelectromagnetic waves while the hydrogen molecules that are remaining inthe optical fiber is not lower than 1×10¹⁶ molecules/cm³, so as toimpart the characteristic that when 10⁸ pulses of KrF excimer laserhaving a wavelength of 248 nm are applied to the optical fiber, asultraviolet ray irradiation for an evaluation of ultraviolet resistance,at an output of 10 mJ/cm² and a pulse frequency of 100 Hz, the opticalfiber as measured with a sample of one meter length has a transmittanceof not less than 90% of the transmittance measured prior to theirradiation at a wavelength of 248 nm.

The process can further include after the step (3), a step of (4)reducing the hydrogen molecules that are remaining in the optical fiberto not higher than 1×10¹⁶ molecules/cm³ by allowing to stand in theatmosphere or by heating at 80° C. or lower. The core of the opticalfiber can include high-purity silica glass containing fluorine. The coreof the optical fiber can include high-purity silica glass which containsOH groups in an amount of not less than 100 ppm and which does notcontain more than 1 ppm of Cl. The high-purity silica glassadvantageously further contains OH groups in an amount less than 100ppm. The optical fiber can be a bundle fiber formed by bundling aplurality of glass fibers.

That is, according to the present invention, there can be provided anoptical fiber having the characteristic that when 10⁸ pulses of KrFexcimer laser having a wavelength of 248 nm are applied to the opticalfiber, as ultraviolet ray irradiation for an evaluation of ultravioletresistance, at an output of 10 mJ/cm² and a pulse frequency of 100 Hz,the optical fiber as measured with a sample of one meter length has atransmittance of not less than 90% of the transmittance measured priorto the irradiation in a wavelength region of ultraviolet ray from 160 nmto 300 nm.

Furthermore, according to the present invention, there can be providedan optical fiber having the characteristic that when 10⁸ pulses of KrFexcimer laser having a wavelength of 248 nm are applied to the opticalfiber, as ultraviolet ray irradiation for an evaluation of ultravioletresistance, at an output of 10 mJ/cm² and a pulse frequency of 100 Hz,the optical fiber as measured with a sample of one meter length has atransmittance of not less than 90% of the transmittance measured priorto the irradiation at a wavelength of 248 nm.

In a preferred embodiment, the electromagnetic waves for use in step (3)is KrF or ArF excimer laser. When the KrF laser beam is used in step(3), the irradiation is preferably performed in such a manner that 10⁶to 10⁷ pulses are applied at 1 mJ/cm²/pulse to 200 mJ/cm²/pulse.Moreover, step (4) is performed in such a manner that the silica glassarticle is heated at a temperature from room temperature to 80° C.

In another preferred embodiment of the present invention, the opticalfiber comprises a core comprising a high purity silica glass, and a cladcomprising a high purity silica glass containing fluorine. In addition,the optical fiber according to the present invention preferablycomprises a core comprising a high purity silica glass and containingsubstantially no dopant for adjusting refractive index.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the transmission spectra, as normalized by the initialtransmission spectrum, of a silica glass article according to Example 1after it has been subjected to the respective treatments.

FIG. 2 shows the transmission spectra, as normalized by the initialtransmission spectrum, of a silica glass article according to Example 5after it has been subjected to the respective treatments.

FIG. 3 is a graph showing the relationship between dose of appliedradiation and a residual elongation of a coating resin.

FIG. 4 is a diagram showing a plurality of examples of defects in glass.

FIG. 5 is a graph showing an emission spectrum of a deuterium lamp.

FIG. 6(a) is a graph showing changes in the concentration of hydrogenwith time at the center of an optical fiber having a diameter of 125 μmupon doping the optical fiber with hydrogen at various temperatures.

FIG. 6(b) is a graph showing changes in the concentration of hydrogenwith time at the center of an optical fiber having a diameter of 125 μmin the stage where hydrogen is discharged from the optical fiber atvarious temperature.

FIG. 7(a) is a graph showing changes in the concentration of hydrogenwith time at the center of an optical fiber having a diameter of 200 μmupon doping the optical fiber with hydrogen at various temperatures.

FIG. 7(b) is a graph showing changes in the concentration of hydrogenwith time at the center of an optical fiber having a diameter of 200 μmin the stage where hydrogen is discharged from the optical fiber atvarious temperature.

FIG. 8 shows the transmission spectra of an optical fiber that has beensubjected to the respective steps and the ultraviolet ray resistancetest in Example 12.

FIG. 9 is an explanatory drawing typically showing step (b) according tothe present invention.

FIG. 10 is a schematic cross-sectional drawing showing the mono-corefiber used in examples according to the present invention.

FIG. 11 is a schematic explanatory drawing showing the structure of thebundle fiber used in examples according to the present invention.

FIG. 12 is an explanatory drawing typically showing the process ofdeuterium lamp irradiation in Comparative Example 3 in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, a silica glass article which is used as a rawmaterial is irradiated with electromagnetic waves under specificirradiation condition so as to convert all of precursors which can beformed into defects in glass (hereinafter sometimes simply referred to“defects”) into defects (step (1)), and then subjected to a hydrogentreatment (step (2)). A silica glass article thus obtained does notdeteriorate any longer in subsequent ultraviolet ray irradiation.

According to the present invention, if step (3) is performed, namely ifthe silica glass article that has been subjected to step (2) isirradiated again with electromagnetic waves while hydrogen moleculesremain therein, the effects obtained by steps (1) and (2) are furtherensured.

Also, the silica glass article subjected to step (3) may be subjected tostep (4) in which the silica glass article is heated to remove hydrogenmolecules from the glass.

As described earlier, adjustment of the contents of OH groups, F and Clas disclosed in Document (1) is effective to improve the initialtransmission characteristic but is not effective for preventing itsdeterioration caused by ultraviolet ray irradiation. On the other hand,the hydrogen treatment disclosed in Document (2) can improve the initialresistance against radiation but cannot prevent increase in lightabsorption for a sufficient period of time. According to the results ofexperiments performed on optical fibers by the inventors of the presentinvention, similar results were also obtained with respect todeterioration due to ultraviolet ray irradiation.

The reason for this is considered to be as follows. Hydrogen moleculesare dispersed in glass as a result of the hydrogen treatment and thenbonded with defects in glass generated by irradiation, to therebyprevent increase in absorption of ultraviolet rays. However, thehydrogen molecules are usually dispersed if the glass is allowed tostand at room temperature. As a result, hydrogen molecules aredischarged from the glass in about two months. Thus, the effect ofpreventing deterioration due to ultraviolet ray irradiation becomeslost.

To prevent deterioration due to ultraviolet ray irradiation for a longtime by the process disclosed in Document (2), a countermeasure must betaken in which the hydrogen treatment is repeated or hermetic coating orthe like is performed to prevent dispersion and discharge of thehydrogen. However, there arises a problem in that such a repeatedtreatment makes it impossible to continuously use a fiber, and thatapplying such a hermetic coating is disadvantageous for productivity.

Document (2) discloses that the loss and deterioration resistance in thevisible ray region may sometimes be prevented if a base member oftransparent glass which is not the fiber is subjected to heat drawingand irradiation with radiation prior to the hydrogen treatment. Althoughexperiments were performed, no effect was attained to prevent increasein the loss in the ultraviolet ray region. That is, if the drawingprocess is performed after the transparent glass perform has beensubjected to the above-mentioned treatments, the effect obtainable fromthe previous process cannot be obtained.

Document (3) has reported experiments in which a hydrogen treatment isconducted and then an ultraviolet ray irradiation is performed toconvert existing precursors into defects so as to be bonded to hydrogendispersed by the previous step, followed by a further hydrogen treatmentand a heating treatment. However, the above-mentioned process is toocomplicated because the hydrogen treatment must be performed two times.

On the other hand, the effect of preventing deterioration due toultraviolet ray irradiation according to the process disclosed inDocument (4) was investigated. As a result, the inventors of the presentinvention have found that an undesirable increase not smaller than 30dB/km takes place if an optical fiber is previously irradiated withradiation in a great dose not smaller than 10⁵ Gy.

Irradiation of radiation not smaller than 10⁵ Gy encounters a difficultythat a large amount of optical fibers cannot be processed at a timebecause of limitation of the irradiation condition. Therefore,satisfactory productivity cannot be realized. Another fact was foundthat there arises a problem in that the mechanical strength of the fiberdeteriorates because the ultraviolet curing resin employed to coat ausual optical fiber deteriorates when it is irradiated with radiation.

Also the process disclosed in Document (5) uses irradiation with a largedose, and hence suffers from the problem of the increases in the lossand deterioration of coatings. According to the experiments by theinventors of the present invention, the process disclosed in Document(5) was not suitable for manufacturing the optical fiber for ultravioletrays.

The inventors of the present invention made the following hypothesis.That is, the hydrogen treatment in the conventional process changesinitial defects related to E′ centers and B₂ centers into stable Si—Hbonds. However, no process is performed to prevent E′ centers newlygenerated from precursors because of ultraviolet ray irradiation afterthe hydrogen treatment. Thus, conversion of the newly generated E′centers into Si—H bonds cannot be performed, and absorption ofultraviolet rays increases. As a result of experiments based on suchhypothesis, the present inventors have found that performing a“pre-treatment (defect-generating treatment) prior to a hydrogentreatment” is very effective to prevent increase in the loss anddeterioration due to ultraviolet ray irradiation. As a result of variousinvestigations, a surprising fact was found that the above describedpre-treatment is effective in case where electromagnetic waves areapplied in not such a great quantity of doses as disclosed in Documents(4) and (5). Thus, the present invention was completed.

That is, the present invention is arranged in such a manner that asilica glass article is first irradiated with a predetermined quantityof electromagnetic waves and then subjected to a hydrogen treatment,contrary to the processes disclosed in Documents (2) and (3).

The pre-treatment according to the present invention enables an opticalfiber to be free from increase in the absorption of ultraviolet rays andenables the ultraviolet curing resin or the like employed as the coatingto be free from deterioration. In addition, two times of hydrogentreatment as employed in the process disclosed in Document (3) are notnecessitated. Therefore, advantages can be obtained in terms of the costfor apparatuses and the time required to manufacture the optical fiber.As a matter of course, the manufacturing cost can be reduced.

With regard to the considerable difference in the dose to obtain theabove-mentioned effect between the glass article for ultraviolet raysaccording to the present invention and the fiber for visible raydisclosed in Documents (4) and (5), the present inventors consider it asattributed to the difference in terms of defects in glass affecting thedeterioration.

As to the glass article for ultraviolet rays, defects which causeabsorption in the wavelength region not longer than 300 nm must beremoved. Si• defects cause absorption at 215 nm, and its precursors at163 nm and 245 nm, respectively. Therefore, a key of the presentinvention is reduction of the Si• (E′ center) and its precursors,specifically, a process of removing the precursors and a process ofterminating the defects by hydrogen or the like so as to render thedefects harmless.

Precursors can be removed by: (a) a process in which glass is brought toan oxygen-rich state to prevent generation of Si—Si bonds so as tocontrol the composition in such a manner that Si• defects are decreasedand SiO• defects are generated; and (b) a process in which the number ofSi—Cl bonds is decreased because Si—Cl is dissociated by ultravioletrays. The terminating of defects may be performed by converting Si• intoSi—OH, Si—H or Si—F.

However, generation of Si• could not be decreased when the preliminaryirradiation of electromagnetic waves according to the present inventionwas not performed. The reason for this is considered as attributed tothe structure of glass. That is, precursors, such as Si••OSi and Si—Si,are considered to be inactive and therefore unable to react withhydrogen. Therefore, it is necessary to break these bonds beforehand.The inventors of the present invention have found that as an effectivemeans for performing such breakage, it is useful to employ irradiationof electromagnetic waves, such as γ-rays, ultraviolet rays or the likein an intermediate dose or smaller so that bonds of precursors may bebroken but breaking of normal Si—O—Si bonds may be minimized. In thepresent invention, it is essential to perform the hydrogen treatmentfollowing the above-mentioned breaking of precursor bonds.

It is assumed that according to the present invention substantially allof precursors of E′ center such as oxygen deficient defects andstretched Si—O—Si bonds are converted into E′ centers by irradiation ofelectromagnetic waves at a quantity of not more than 10⁴ Gy. Then, ahydrogen treatment of a deuterium treatment is performed so that all ofthe E′ centers are converted into stable Si—H bonds or Si—D bonds. Evenif ultraviolet ray irradiation is performed after hydrogen (deuterium)has been removed from the glass subjected to the hydrogen treatment orthe deuterium treatment, further generation of E′ center can beprevented because no precursor exists in the glass. Thus, the increaseof absorption can be prevented.

As for a fiber for visible rays, defects which cause light absorption ina wavelength not shorter than 400 nm must be removed. The SiO• defecthas a large absorption band in the vicinity of 600 nm, which must berendered to be harmless. In this case, either of the following processesmay be considered: (a) a base member in which the defects are convertedinto Si—OH is manufactured; and (b) SiO• defects are converted intoSi—OH during irradiation with γ-rays and their precursors SiO—Si andSi—O—O—Si is completely broken off by using γ-rays so that all of theforegoing precursors may be converted into Si—OH.

The process disclosed in Document (4) corresponds to the process (b). Torealize complete conversion to Si—OH, irradiation with γ-rays at aquantity of 10⁶ Gy is required. In this case, irradiation with γ-rays ata large dose must be performed, but addition of hydrogen is not alwaysrequired.

The process according to the present invention is described in detailbelow. In the present invention, the ultraviolet ray region is awavelength region from 160 nm to 300 nm unless otherwise specificallyindicated.

The silica glass article which is used as the raw material according tothe present invention includes all of silica glass products which areoptical elements, such as optical fibers, lenses, beam splitters and thelike, made of silica glass required to industrially use ultravioletrays. The process of manufacturing a silica glass material for thesilica glass article to be subjected to the process of the presentinvention is not particularly limited.

The silica glass having a high purity for use in the present inventionmeans glass containing no impurities, such as transition metal (i.e.,Fe, Cu. Ni and the like), alkali and alkali earth metal. Each of theimpurity elements, if contained, must be no more than the order of PPB.

With regard to specific materials of the silica glass article, the maincomponent of which is silica (SiO₂) and it preferably contains OH groupsat a concentration not lower than 100 ppm nor higher than 2000 ppm,particularly, in the regions through which ultraviolet rays are allowedto pass. The above-mentioned OH-group content of 2000 ppm issubstantially an upper limit of the concentration of OH groups which canbe added to glass by a usual process of manufacturing silica glass, suchas a soot method or a direct method.

In some cases, fluorine (F) is preferably contained in an amount ofabout 1 wt %. As a dopant for changing refractive index, only F may beemployed. If F is contained, the OH-group content may be lower than 100ppm (including a OH-group content of 0 ppm).

It is significantly preferred that the amount of Cl in the core of theoptical fiber or the like must be lower than 1 ppm (a case in which Clis 0 ppm included). On the other hand, the material of a region of theoptical fiber, for example, the clad, through which ultraviolet rays donot pass is free from the above-mentioned limitation.

The constitution of the refractive index distribution of the opticalfiber according to the present invention is not limited particularly.The structure may be any one of a mono-core structure, a multi-corestructure, a single-mode structure and a multi-mode structure. Moreover,a bundle fiber formed by bundling a plurality of optical fibers is alsoincluded in the scope of the present invention. The bundle fiber may beformed in such a manner that the process of the present invention isapplied to each of optical fibers and then the optical fibers are wound.Alternatively, a bundle fiber may be first formed by bundling aplurality of optical fibers, and then the present invention may beapplied to the bundle fiber.

The raw material silica glass article is initially subjected toelectromagnetic wave irradiation treatment. The electromagnetic wavesaccording to the present invention have photon energy with which defectscan be generated when irradiation with ultraviolet rays (400 nm to 160nm), vacuum ultraviolet rays (shorter than 160 nm to 1 nm), X rays(several tens of nm to 0.01 nm) or γ-rays (shorter than 0.01 nm) isperformed. That is, the electromagnetic waves has photon energy of 3.5eV or greater. The upper limit of the energy is generally 1.33 MeV whichis the same as γ-ray energy of ⁶⁰Co which is widely used as a γ-raysource in the industrial field. The foregoing value is a valuedetermined practically.

The dose of rays to be applied is generally 10 to 10⁴ Gy, preferably10²Gy to 10³ Gy. Thus, the above-mentioned low dose is enough to obtaina satisfactory effect of preventing the deterioration due to ultravioletray irradiation.

As for specific irradiation means, in the case of γ-rays ⁶⁰CO, ¹³⁷Cs orthe like is used, and in the case of X-rays an X-ray tube equipped withW, Cu or the like as its target is used, and in the case of ultravioletrays or vacuum ultraviolet rays a deuterium lamp, KrF excimer laser, ArFexcimer laser, synchrotron orbital radiation or the like is used.

After the electromagnetic wave irradiation treatment is performed, thehydrogen treatment is performed in the present invention. The“atmosphere comprising a hydrogen gas” according to the presentinvention is an atmosphere composed of a pure hydrogen gas or a mixedatmosphere of a hydrogen gas, a nitrogen gas and/or inert gas in whichthe partial pressure of the hydrogen gas is generally from 0.1 atm to 10atm, preferably from 0.5 atm to 10 atm. The reason why the pressure isdetermined as described above is as follows. If the pressure is 0.5 atmto 10 atm, substantially similar dispersion speed of hydrogen in theglass can be obtained. Moreover, the gas pressure within the above rangecan be easily employed when an actual production operation is performed.If the pressure exceeds 10 atm, the gas must be treated as ahigh-pressure gas, which is strictly regulated by laws and thus isdisadvantageous in an economical viewpoint. Although a similar effectcan be obtained even if the pressure is about 0.1 atm, a gas of such apressure cannot be easily used practically.

If a deuterium gas is employed as the hydrogen gas, a similar effect canbe obtained.

The temperature at which the hydrogen treatment is performed is notlimited particularly. Hydrogen at a pressure of 1 atm requires aboutseven days to reach the center of the fiber when the temperature is roomtemperature. When the temperature is 80° C., one day is required. Whenthe temperature is 200° C., about two hours are required. Therefore, atemperature of not lower than room temperature may be used. In case ofan optical fiber, an upper limit of the treating temperature issubstantially determined depending on the heat resistance of a coatingfor the fiber. In such a case, the temperature is preferably about 80°C. to 200° C. 80° C. is a temperature close to the highest temperaturethat does not damage ultraviolet curing acrylate resins and 200° C. isthe heat resisting upper limit temperature of silicone resins. Thehydrogen treating time varies depending on the hydrogen treatingtemperature. However, if the treating temperature is not lower than 80°C., hydrogen can be dispersed in the fiber within about two or threedays to complete the treatment.

The electromagnetic wave treatment and the hydrogen treatment accordingto the present invention are performed so that increase in lightabsorption in the ultraviolet ray region occurring due to ultravioletray irradiation is substantially prevented. The expression“substantially prevented” used herein means that deterioration in thetransmission occurring due to ultraviolet ray irradiation is not higherthan 10% of the initial transmission. That is, when an initialultraviolet ray transmission (initial transmission) is T₀, anultraviolet ray transmission after irradiated with ultraviolet rays (160nm to 300 nm) is T₁, and the relative transmission after the irradiationregarding T₀ as 100% is T_(R) (T_(R)=T₁/T₀×100 (%) ), a relationship1−T_(R)≦10%, that is, {(T₀−T₁)/T₀}≦10% is satisfied.

According to the present invention, an optical fiber is considered freefrom substantial increase in light absorption due to ultraviolet rayirradiation in the ultraviolet ray region when the decrease in theultraviolet ray transmission is not more than 10% in a wavelength regionof ultraviolet ray from 160 nm to 300 nm as measured with a sample ofone meter length after the following irradiation is applied for anevaluation:

Types of laser: KrF excimer laser

Wavelength: 248 nm

Number of pulse: 10⁸ pulses

Energy density: 10 mJ/cm²

Pulse frequency: 100 Hz

In case where a silica glass article obtained by the process accordingto the present invention which is substantially free from increase inlight absorption in the ultraviolet ray region occurring due toultraviolet ray irradiation is an optical fiber having a length of 1meter, and is irradiated in such a manner that 10⁸ pulses of KrF excimerlaser having a wavelength of 248 nm are applied under conditions thatthe output is 10 mJ/cm² and the pulse frequency is 100 Hz, the silicaglass article has a difference between the initial ultraviolet raytransmission and the ultraviolet ray transmission after irradiated withultraviolet rays not higher than 10% of the initial transmission in awavelength region of ultraviolet rays from 160 nm to 300 nm.

If the silica glass article according to the present invention is anoptical fiber, thermosetting silicones or ultraviolet curing urethaneacrylates can be used as a primary coating, and nylons can be used as asecondary coating. In the present invention, ultraviolet curing urethaneacrylates are preferably used as the primary coating because they have ahigh residual elongation ratio after irradiated with radiation. FIG. 3shows the relationship between the dose of the applied radiation and theresidual elongation ratio (the ratio of breaking elongation after theirradiation with respect to breaking elongation before the irradiation)of the resin. As can be understood from FIG. 3, deterioration starts ifthe dose exceeds 10⁵ Gy and deterioration in the coating cansubstantially be prevented if the dose is 10⁴ Gy or smaller which is theirradiation condition according to the present invention.

As described above, the present invention has step (1) in which unstablestructures, such as oxygen-deficient type defects, in the silica glassis forcibly bonded with each other and step (2) in which the portionthus broken off is forcibly bonded with hydrogen. Hydrogen moleculesintroduced into a silica glass article in the form of a plate-like shapeor a block shape having a relatively large size remain for a relativelylong time (several years) therein. Therefore, ultraviolet rays areapplied in a state in which hydrogen molecules remain in the silicaglass so that the ultraviolet ray resistance is maintained.

In contrast, if the silica glass article has a small size with respectto the diffusion coefficient of hydrogen in the glass, hydrogenmolecules introduced into the silica glass dissipate therefrom in arelatively short period of time. For example, if the outer diameter ofthe fiber is 125 μm, hydrogen molecules get out within 8 to 9 weeks. Ifthe outer-diameter is 200 μm, hydrogen molecules get out within about 24weeks.

FIG. 6(a) shows calculated changes in the concentration of hydrogen atthe center of a glass fiber having an outer diameter of 125 μm in thecourse of hydrogen doping at from 20° C. to 80° C. In FIG. 6(a), theinitial concentration is shown as 0 and the saturated concentration isshown as 1. The glass fiber is surrounded with the atmosphere, thehydrogen partial pressure of which is regarded as 1 atm. FIG. 6(b) alsoshows calculated changes in the concentration of hydrogen in the coursewhen hydrogen dissipates from the fiber. The glass fiber is surroundedwith the atmosphere, the hydrogen partial pressure of which is regardedas zero.

In FIGS. 7(a) and (b), calculated changes in hydrogen concentration in aglass fiber having an outer diameter of 200 μm are shown in the samemanner as in FIGS. 6(a) and (b).

With regard to such dissipation of hydrogen, similar results wereobtained with the fibers manufactured by the conventional techniquesdisclosed in Documents (6) to (12).

It was found that there may be a case where a transmission loss takesplace when irradiated with ultraviolet rays, even if steps (1) and (2)according to the present invention are performed. Although the reasonfor this has not been detected, the present inventors have considered asfollows. By steps (1) and (2), moderate bonds are formed between glassand hydrogen. The bonds enables the ultraviolet ray resistance of thesilica glass article to be maintained to some extent even after surplushydrogen gets out therefrom. However, if the silica glass article isallowed to stand for a long time after step (2), hydrogen which isbonded to glass with the relatively moderate bond is eventuallyreleased. As a result, the ultraviolet ray resistance deteriorates.

The problem of the hydrogen dissipation can be solved by step (3) of thepresent invention in which an electromagnetic wave treatment isperformed in a state in which hydrogen molecules remain in the silicaglass article.

As the “state in which hydrogen molecules remain in the glass” when step(3) is performed, the concentration of hydrogen molecules in silicaglass is preferably 10¹⁶ molecules/cm³ or higher, more preferably 10¹⁶molecules/cm³ to 10²⁰ molecules/cm³, and most preferably 10¹⁸molecules/cm³ to 10²⁰ molecules/cm³.

The electromagnetic waves for use in step (3) of the present inventionare preferably ultraviolet rays having a wavelength of 248 nm orshorter, more preferable excimer laser beams or γ-rays, and mostpreferably KrF excimer laser beams or ArF excimer laser beams. Theconditions under which the electromagnetic waves are applied in step (3)is that the amount of irradiation is 1 to 200 mJ/cm²/pulse and 10⁶ to10⁷ pulses (about 2 hours to 3 hours in terms of period of time) areapplied when KrF excimer laser beam is employed. When the ArF excimerlaser beam is employed, the conditions are that the amount ofirradiation is 1 to 200 mJ/cm²/pulse and 10⁴ to 10⁷ pulses. Thefrequency of the pulses is, for example, 50 Hz to about 1000 Hz. Theconditions are not limited to the above-mentioned values. Therefore,practical conditions which can be employed may be selected.

Although the mechanism of step (3) has not been clarified, the inventorsof the present invention consider that irradiation with electromagneticwaves such as excimer laser beams enhances bonding between hydrogen anddefects and thus the bonding is converted to further stable bond. Thatis, hydrogen is brought to a fixed state so that removal of hydrogen isprevented. The reason why hydrogen can be fixed by irradiation withexcimer laser beams in about from 2 to 3 hours is considered that incase of excimer laser beams, strong pulses can instantaneously andconcentrically be applied to the end surface of the fiber and thusenergy can efficiently be used so that hydrogen is fixed. The fixationof hydrogen in step (3) is advantageous for a silica glass articlehaving a small size, particularly, an optical fiber.

A silica glass article subjected to step (3) of the present inventioncauses substantially no increase in its light absorption in theultraviolet ray region, if irradiated with ultraviolet rays. Forexample, in a case where 10⁸ pulses of KrF excimer laser having awavelength of 248 nm are applied to an optical fiber, as a ultravioletray irradiation for an evaluation of ultraviolet resistance, at anoutput of 10 mJ/cm² and a pulse frequency of 100 Hz, the optical fiberas measured with a sample of one meter length has a transmittance of notless than 90% of the transmittance measured prior to the irradiation.

After hydrogen has been fixed by performing irradiation withelectromagnetic waves in step (3), there may be a case where unfixedhydrogen molecules still remain in the glass. Although existence of thehydrogen molecules does not raise any problem when the glass is used ina short wavelength region, an adverse result occurs in a long wavelengthregion in which absorption by hydrogen molecules takes place. Thehydrogen molecules (H₂) has an absorption band at 1.24 μm and theabsorbing strength is 3.4 dB/km with the concentration of 1×10¹⁸molecules/cm³ and 0.03 dB/km with the concentration of 1×10¹⁶molecules/cm³.

To remove the unfixed hydrogen molecules in the glass, there may beemployed a heating treatment as step (4). It is preferred that theheating is performed at a temperature of, for example, from roomtemperature to 80° C., so as to make the concentration of hydrogenmolecules remaining in the silica glass article that has been subjectedto step (4) to be 1×10¹⁶ molecules/cm³ or lower. The unfixed hydrogencan also be removed by merely allowing the silica glass article to standin the atmosphere.

The value of hydrogen concentration in the silica glass can be obtainedby Raman analysis disclosed in Zurnal Pril; adnoi Spektroskopii Vol. 46No. 6 pp. 987-991 June 1987 (Document (13)) in accordance with theequation about the intensity ratio of the intensity of a Raman band ofSiO₂ having a wavelength of 800 cm⁻¹ and that of hydrogen molecules inthe synthesis silica glass at a wavelength of 4135 cm⁻¹.

A silica glass article subjected to step (4) of the present inventioncauses substantially no increase in its light absorption in theultraviolet ray region, if irradiated with ultraviolet rays. Forexample, in the case where an optical fiber having a length of 1 m isirradiated in such a manner that 10⁸ pulses of KrF excimer laser havinga wavelength of 248 nm are applied under condition that the output is 10mJ/cm² and the pulse frequency is 100 Hz, the silica glass article(i.e., optical fiber) exhibits a difference between the initialultraviolet ray transmission and the ultraviolet ray transmission afterirradiated with ultraviolet rays is not higher than 10% of the initialtransmission at a wavelength of 248 nm.

The present invention will be described in detail with reference to thefollowing Examples and comparative Examples, but the invention shouldnot be construed as being limited thereto.

EXAMPLE 1

One meter length of an optical fiber composed of: a high-purity silicaglass core containing OH groups in an amount of 1000 ppm and Cl in anamount lower than 1 ppm; and a silica glass clad (relative refractiveindex difference Δn=1.0) to which fluorine was added (in an amount of 3wt %) was, from both ends thereof, irradiated by a deuterium lamp for 24hours. Assuming that the refractive index of the core is n_(core) andthe refractive index of the clad is n_(clad), the relative refractiveindex difference Δn can be expressed by the equation{Δn=(n_(core)−n_(clad))/n_(core)}. The deuterium lamp had the wavelengthspectrum shown in FIG. 5 and a broad light emission peak in the vicinityof a wavelength of 230 nm (5.4 eV). The transmittance (initialtransmittance T₀) before irradiation using the deuterium lamp andtransmittance (T₁) after subjected to the irradiation were measured inthe wavelength region from 200 nm to 450 nm (measuring apparatus:Instantaneous Multi-Photometry System Model No. MCPD-200 manufactured byOtuka Electronics Co.). The transmission characteristics in the overallregion from a wavelength of 200 nm to 450 nm are shown in FIG. 1, in theform of relative transmittance T_(R) with respect to the initialtransmittance which is regarded as 1.00 for each wavelength, with asolid line having marks x. It can be understood that defects in glasswere generated in the optical fiber as a result of the irradiation andthe transmittance deteriorated.

The optical fiber was immediately allowed to stand in an atmosphere ofhydrogen gas (H₂), the pressure of which was 1 atm, for one week. As aresult, ultraviolet ray transmission characteristics (relative changewith respect to the initial transmittance similar to the above) asindicated in FIG. 1 with a solid line having marks o was obtained. Lightabsorption due to defects in glass became extinct.

The ultraviolet ray transmission characteristics of the above opticalfiber after irradiated again with the deuterium lamp for 24 hours was asindicated with a solid line having square marks shown in FIG. 1. Thetransmission characteristic was the same as that realized immediatelyafter the hydrogen treatment. In the wavelength region from 200 nm to300 nm, no increase in the light absorption by irradiation was observed.As a result, it can be understood that deterioration does not occurafter the optical fiber is subjected to the process according to thepresent invention, even if ultraviolet ray irradiation is performed.

EXAMPLE 2

A bundle fiber (having a length of 1 m) was manufactured by bundling onehundred optical fibers each composed of: a high-purity silica glass corecontaining OH groups in an amount of 1000 ppm and Cl in an amount lowerthan 1 ppm; and a silica glass clad (relative refractive indexdifference Δn=1.0) to which fluorine was added (in an amount of 3 wt %).The bundle fiber had both ends fixed by epoxy resin. Then, KrF excimerlaser (wavelength: 248 nm, 5 eV) was applied to both ends of the bundlefiber in the axial direction. The irradiation conditions were 100mJ/cm²/pulse and 10⁶ pulses at 50 Hz. As a result of the irradiation,defects were generated in the optical fibers and a reduction in thetransmittance was greater than that realized in Example 1 shown in FIG.1. Then, the optical fiber was immediately exposed to a hydrogen gasatmosphere having a temperature of 80° C. and a pressure of 5 atm, forone week. As a result, the resulting relative transmittance of 1.00 wasrealized and light absorption due to defects in the glass became extinctas in the case of Example 1. Furthermore, the bundle fiber wasirradiated by the deuterium lamp for 48 hours. As a result, no increasein the light absorption by the irradiation was observed in thewavelength region from 200 nm to 300 nm. Moreover, irradiation with KrFexcimer laser was performed under conditions of 100 mJ/cm²/pulse and 10⁶pulses at 50 Hz. No increase in the light absorption was observed in thewavelength region from 200 nm to 300 nm.

EXAMPLE 3

An optical fiber (having a length of 1000 m) each composed of: ahigh-purity silica glass core containing OH groups in an amount of 2000ppm and Cl in an amount lower than 1 ppm; and a silica glass clad towhich fluorine was added, was wound. Then, γ-rays (photon energy of 1.17MeV and 1.33 MeV), the source of which was ⁶⁰Co were applied to theoverall body of the wound optical fibers. The dose of radiation absorbedby the fiber was 10³ Gy. As a result of the irradiation, defects weregenerated in the optical fibers and a reduction in the transmittance wasgreater than that realized in Example 1. Then, the optical fiber wasimmediately exposed to a hydrogen gas atmosphere having a temperature of80° C. and a pressure of 5 atm, for one week. As a result, the lightabsorption occurring due to the defects became extinct and the relativetransmittance returned to 1.00. The fiber was cut to a length of 1 m,and then irradiated with the deuterium lamp for 48 hours. As a result,no increase in the light absorption was observed in the wavelengthregion from 200 nm to 300 nm. Moreover, irradiation with KrF excimerlaser was performed under conditions of 100 mJ/cm²/pulse and 10⁶ pulsesat 50 Hz. No increase in the light absorption was observed in thewavelength region from 200 nm to 300 nm.

EXAMPLE 4

A high-purity silica glass plate (30 mm×30 mm×5 mm) containing OH groupsin an amount of 2000 ppm and Cl in an amount lower than 1 ppm wasirradiated with γ-rays (photon energy of 1.17 MeV and 1.33 MeV), thesource of which was ⁶⁰Co. The dose of radiation absorbed by the fiberwas 10³ Gy. As a result, defects were generated in the high-puritysilica glass plate and great reduction in the transmittance wasobserved. The high-purity silica glass plate was immediately exposed toa hydrogen gas atmosphere having a temperature of 200° C. and a pressureof 10 atm, for one week. As a result, the light absorption occurring dueto the defects became extinct. The high-purity silica glass plate wasirradiated with KrF excimer laser of 10 mJ/cm²/1 pulse and 10⁴ pulses at50 Hz. No increase in the light absorption was observed in thewavelength region from 170 nm to 300 nm.

EXAMPLE 5

An optical fiber (having a length of 1 m) composed of: a high-puritysilica glass core containing OH groups in an amount of 500 ppm and Cl inan amount of 300 ppm; and a silica glass clad to which fluorine wasadded, was irradiated by a deuterium lamp from both ends thereof in theaxial direction for 24 hours. As a result, defects were generated in theoptical fiber so that a light absorption characteristic as shown in FIG.2 was realized. The optical fiber was immediately exposed to a hydrogengas atmosphere having a room temperature and a pressure of 1 atm, forone week. As a result, light absorption occurring due to the defectsbecame extinct completely. The fiber was again irradiated by thedeuterium lamp for 24 hours. As a result, increase in the lightabsorption was observed in the wavelength region from 200 nm to 300 nm.The results were shown in FIG. 2 in the same manner as in FIG. 1.

Comparative Example 1

An optical fiber preform composed of a fluorine-doped silica glass cladand a high-purity silica glass core containing OH groups in an amount of2000 ppm and Cl in an amount lower than 1 ppm was manufactured. Theoptical fiber preform was previously irradiated with 10³ Gy γ-rays.Then, the optical fiber preform was drawn so that an optical fiberhaving a diameter of 200 μm was manufactured. The resulting opticalfiber having a length of 1 m was exposed to a H₂ atmosphere at 80° C.and 5 atm for one week. The fiber was irradiated with 10⁶ pulses of KrFexcimer laser at 100 mJ/cm²/pulse at 50 Hz. As a result, increase in thelight absorption was observed in the wavelength region from 200 nm to300 nm. As a result, a fact can be understood that no effect is obtainedeven if the glass optical fiber preform is irradiated with γ-rays.

EXAMPLE 6

The fiber having an excellent ultraviolet ray characteristic andaccording to Example 3 was again irradiated with 10⁶ Gy γ-rays. Increasein a loss not smaller than 100 dB/km took place at 500 nm. As a result,an effect of the limitation according to the present invention that thedose should be 10⁴ Gy or smaller can be confirmed.

EXAMPLE 7

An optical fiber (having a length of 1000 m) each composed of afluorine-doped silica glass clad and a high-purity silica glass corecontaining OH groups in an amount of 2000 ppm and Cl in an amount lowerthan 1 ppm, was wound. The wound optical fiber was posited one meterfrom an X-ray tube having a W (tungsten) target and was irradiated withX-rays for one hour at the applied voltage of 50 kV and tube current of80 mA. As a result, defects were generated in the optical fibers and areduction in the transmittance of the same degree with that in Example 1was observed. The optical fiber was immediately exposed to a hydrogenatmosphere, the temperature of which was 80° C. and the pressure ofwhich was 5 atm for one week. As a result, light absorption occurringdue to the defects became extinct, and the relative transmittancereturned to 1.00. The fiber was cut to a length of 1 m, and thenirradiated with the deuterium lamp for 48 hours. As a result, noincrease in the light absorption was observed in the wavelength regionfrom 200 nm to 300 nm. Moreover, irradiation with KrF excimer laser wasperformed under conditions of 100 mJ/cm²/pulse and 10⁶ pulses at 50 Hz.No increase in the light absorption was observed in the wavelengthregion from 200 nm to 300 nm.

EXAMPLE 8

Optical fibers (having a length of 1000 m) each composed of afluorine-doped silica glass clad and a high-purity silica glass corewhich contained OH groups in an amount of 300 ppm and Cl in an amountlower than 1 ppm and which was doped with fluorine (1 wt % ), werewound. Then, γ-rays (photon energy of 1.17 MeV and 1.33 MeV), the sourceof which was ⁶⁰Co were applied to the overall body of the wound opticalfibers. The dose of radiation absorbed by the fiber was 10³ Gy. As aresult, defects were generated in the optical fibers and a reduction inthe transmittance was greater than that realized in Example 1. Then, theoptical fiber was immediately exposed to a hydrogen gas atmospherehaving a temperature of 80° C. and a pressure of 5 atm, for one week. Asa result, the light absorption occurring due to the defects becameextinct and the relative transmittance returned to 1.00. The fiber wascut to a length of 1 m, and then irradiated with the deuterium lamp for48 hours. As a result, no increase in the light absorption was observedin the wavelength region from 200 nm to 300 nm. Moreover, irradiationwith KrF excimer laser was performed under conditions of 100mJ/cm²/pulse and 10⁶ pulses at 50 Hz. No increase in the lightabsorption was observed in the wavelength region from 200 nm to 300 nm.

EXAMPLE 9

The same procedure as in Example 8 was followed, except that only thewavelength of the excimer laser and the irradiation condition in Example8 were changed. Irradiation with ArF excimer laser was performed at 10mJ/cm²/pulse and 10⁴ pulses at 50 Hz. No increase in the lightabsorption was observed in the wavelength region from 170 nm to 300 nm.

EXAMPLE 10

Optical fibers (having a length of 1000 m) each composed of afluorine-doped silica glass clad and a high-purity silica glass corewhich contained OH groups in an amount of 300 ppm and Cl in an amountlower than 1 ppm and which was doped with fluorine (1 wt %), were wound.Then, γ-rays (photon energy of 1.17 MeV and 1.33 MeV), the source ofwhich was ⁶⁰Co were applied to the overall body of the wound opticalfibers. The dose of radiation absorbed by the fiber was 10⁶ Gy. As aresult, defects were generated in the optical fibers and a reduction inthe transmittance was greater than that realized in Example 1. Then, theoptical fiber was immediately exposed to a hydrogen gas atmospherehaving a temperature of 80° C. and a pressure of 5 atm, for one week. Asa result, the light absorption occurring due to the defects becameextinct and the relative transmittance returned to 1.00. The fiber wascut to a length of 1 m, and then irradiated with the deuterium lamp for48 hours. As a result, no increase in the light absorption was observedin the wavelength region from 200 nm to 300 nm. Moreover, irradiationwith KrF excimer laser was performed under conditions of 10 mJ/cm²/pulseand 10⁸ pulses at 50 Hz. No increase in the light absorption wasobserved in the wavelength region from 200 nm to 300 nm. In a numericalviewpoint, the optical fiber according to this example has thedifference of 10% or lower between the initial transmittance ofultraviolet rays and the transmittance of ultraviolet rays afterultraviolet ray irradiation was performed.

In each of the above Examples, irradiation by using the deuterium lampwas performed in such a manner that the output from the lamp was 150 Wand the distance from the lamp to the hydrogen was 15 cm. Theirradiation was conducted from both ends of the optical fiber in theaxial direction of the optical fiber.

EXAMPLE 11

Optical fibers were manufactured by processing in the same manner as inExamples 1 to 10 and comparative Example 1, respectively. Then, thehydrogen molecule concentrations in each optical fiber immediately afterconducting the respective step (2) were obtained by a Raman analysisdisclosed in Document (13). The results were shown in Table 1.

TABLE 1 Hydrogen Molecule Concentration in Silica Glass Fiberimmediately after Step (2) Example No. (molecules/cm³) Example 1 8 ×10¹⁸ Example 2 3 × 10¹⁹ Example 3 3 × 10¹⁹ Example 4 8 × 10¹⁹ Example 53 × 10¹⁸ Comparative 3 × 10¹⁹ Example 6 3 × 10¹⁹ Example 6 3 × 10¹⁹Example 7 3 × 10¹⁹ Example 8 3 × 10¹⁹ Example 9 3 × 10¹⁹ Example 10 3 ×10¹⁹

Examples 12 to 21 in which treatments were conducted up to step (3) orstep (4) according to the present invention and comparative Examples 2to 4 are described below. Irradiation conditions in the ultraviolet rayresistance tests performed to evaluate each optical fiber according toeach example were as follows.

Irradiation with KrF excimer laser: wavelength 248 nm; 5 eV; 10mJ/cm²/pulse; 1000 Hz; and 10⁸ pulses were applied to both ends of theoptical fiber in the axial direction of the optical fiber.

Irradiation with ArF excimer laser: wavelength 193 nm; 6.4 eV; 10mJ/cm²/pulse; 1000 Hz; and 10⁴ pulses were applied to both ends of theoptical fiber in the axial direction of the optical fiber.

Irradiation by deuterium lamps: lamp output 150 W, the distance fromlamp to optical fiber was 15 cm and the optical fiber was irradiatedfrom both ends thereof in the axial direction of the optical fiber.

Irradiation with γ-rays: irradiation source ⁶⁰Co, 1.17 MeV and 1.33 MeV.

The evaluation of the result of the ultraviolet ray resistance test wasmade by comparing a (initial) ultraviolet ray transmittance immediatelybefore the irradiation (transmittance immediately after the completionof the final treatment step) and a ultraviolet ray transmittance afterthe irradiation.

In the ultraviolet ray resistance test, increase in the absorbedultraviolet ray amount in the ultraviolet ray region is determined bythe laser output and the number of applied pulses. The frequency of thepulses is a value which can practically be selected. If a high frequencyis employed, a required amount of irradiation can be realized in a shorttime.

EXAMPLE 12

Optical fibers (having a length of 1000 m) each of which was composedof: a silica glass core containing fluorine in an amount of 1 wt %; anda silica glass clad containing fluorine in an amount of 3 wt %, werewound. The overall body of the bundle was irradiated with γ-rays (photonenergy of 1.17 MeV and 1.33 MeV) the source of which was ⁶⁰Co (step(1)). At this time, the dose of radiation absorbed by the fiber was 10³Gy. The bundle was immediately exposed to a hydrogen atmosphere, thetemperature of which was 80° C. and the pressure of which was 10 atm,for one week (step (2)). At this time, the concentration of hydrogenmolecules in the optical fiber was 7×10¹⁹ molecules/cm³. The fiber wascut to a length of 1 m, and then the optical fiber was irradiated fromboth ends thereof with 10⁷ pulses of KrF excimer laser (wavelength: 248nm, 5 eV) under irradiation condition of 10 mJ/cm²/pulse and 1000 Hz(step (3)). Thus, the optical fiber according to the present inventionwas obtained. The transmittances immediately after the respective stepare shown in FIG. 8. In the figure, the curve referred to (a) is thetransmittance after the step (1), the curve referred to (b) is thetransmittance after the step (2), the curve referred to (c) is thetransmittance after the step (3), and the curve referred to (d) is thetransmittance after the ultraviolet ray resistance test. Thetransmittances shown in FIG. 8 are those expressed by the Equation 1shown below, when light having a wavelength of λ is made incident on anend of an optical fiber at an intensity of I₀ and is emitted throughanother end of the optical fiber at an intensity of I.

Transmittance T(λ)=I(λ)/I ₀(λ)  Equation 1

The ultraviolet ray resistance test was performed in such a manner thatthe obtained optical fiber was irradiated from both ends thereof with10⁸ pulses of KrF excimer laser. The deterioration in transmittance at awavelength of 248 nm was merely such that the transmittance after theirradiation was 96% of the transmittance after the step (3) (before theirradiation for testing). This result can be seen in FIG. 8.

EXAMPLE 13

The same procedure as in Example 12 was followed, except that theabsorption dose of radiation in the step (1) was changed to 10² Gy. Theoptical fiber thus obtained was subjected to a ultraviolet rayresistance test by application of 10⁸ pulses of KrF excimer laser in thesame manner as in Example 12. The results were similar to those obtainedin Example 12.

EXAMPLE 14

One meter length of an optical fiber composed of: a silica glass corecontaining fluorine in an amount of 1 wt %; and a silica glass cladcontaining fluorine in an amount of 3 wt % was subjected to the sametreatment steps (1) to (3) as in Example 12. The dose of radiationabsorbed by the optical fiber in step (1) and the concentration ofhydrogen molecules in the optical fiber realized immediately after step(2) were each the same as those in Example 12. After step (3) wasperformed, the optical fiber was heated to 40° C. for three weeks (about500 hours) so that removal of hydrogen was performed (step (4)) so thatthe optical fiber according to the present invention was obtained. Theconcentration of hydrogen molecules in the thus obtained optical fiberwas not higher than a measurement limit (less than 1×10¹⁶molecules/cm³).

The optical fiber thus obtained was subjected to the ultraviolet rayresistance test by application of 10⁸ pulses of KrF excimer laser in thesame manner as in Example 12. As a result, the deterioration intransmittance at a wavelength of 248 nm was merely such that thetransmittance after the irradiation for testing was 93% of thetransmittance after the step (4).

Comparative Example 2

The same optical fiber (having a length of 1 m) as used in Example 14was subjected to steps (1) and (2) in the same manner as in Example 14.After the step (2) was completed, the optical fiber was, in theatmosphere, heated to 60° C. for 10 days so that Hydrogen was removed.As a result, the concentration of hydrogen molecules in the fibers wasnot higher than the measurement limit (less than 1×10¹⁶ molecules/cm³).

The optical fiber, from which hydrogen was removed, was subjected to theultraviolet ray resistance test by application of 10⁸ pulses of KrFexcimer laser in the same manner as in Examples 12 and 14. As a result,the transmittance at a wavelength of 248 nm was deteriorated to 63% ofthe transmittance immediately after the step (4) (removal of hydrogen).

EXAMPLE 15

The same optical fiber (having a length of 1 m) as used in Example 14was subjected to steps (1) and (2) in the same manner as in Example 14.The concentration of hydrogen molecules in the optical fiber after step(2) was 7×10¹⁹ molecules/cm³. The optical fiber (having a length of 1 m)thus obtained according to the present invention was subjected to theultraviolet ray resistance test by irradiating With the deuterium lampfor 24 hours. As a result, the transmittance at a wavelength of 248 nmshowed almost no changes from the transmittance immediately after thestep (2). Then, the irradiation by the deuterium lamp was continued for3 months, but no change in the transmission characteristic was observed.

EXAMPLE 16

One meter length of an optical fiber which is composed of: a core madeof pure silica glass (SiO2) containing OH groups in an amount of 1000ppm and Cl in an amount lower than 1 ppm; and a silica glass cladcontaining fluorine in an amount of 3 wt %, and which has a length of 1m was irradiated with γ-rays (photon energy of 1.17 MeV and 1.33 MeV),the source of which was ⁶⁰Co, in such a manner that the dose ofradiation absorbed by the fiber was 10² Gy (step (1)). The fiber wasimmediately exposed to a hydrogen atmosphere, the temperature of whichwas 80° C. and the pressure of which was 10 atm, for one week (step(2)). The concentration of hydrogen molecules in the optical fiber atthe completion of the step (2) was 7×10¹⁹ molecules/cm³. Then, theoptical fiber was irradiated from both ends thereof with 10⁷ pulses ofKrF excimer laser (wavelength: 248 nm, 5 eV) under condition of 10mJ/cm²/pulse and 1000 Hz (step (3)).

The optical fiber (having a length of 1 m) thus obtained according tothe present invention was subjected to the ultraviolet ray resistancetest in such a manner that the optical fiber was irradiated by thedeuterium lamp for 24 hours. As a result, however, the transmittance ata wavelength of 248 nm was not almost changed from the transmittanceimmediately after the step (3). Then, the irradiation by the deuteriumlamp was continued for 3 months, but no change in the transmissioncharacteristic was observed.

EXAMPLE 17

One meter of an optical fiber composed of: a silica glass corecontaining fluorine by 1 wt %; and a silica glass clad containingfluorine in an amount of 3 wt % was subjected to steps (1) and (2) inthe same manner as in Example 14. Step (3) was performed in such amanner that the γ-rays which was the same as that used in the step (1)were applied up to an absorption dose of radiation of 10² Gy. Theoptical fiber thus obtained according to the present invention wassubjected to an ultraviolet ray resistance test in such a manner thatthe optical fiber was irradiated from both ends thereof with 10⁸ pulsesof KrF excimer laser in the axial direction of the optical fiber. As aresult, the deterioration in transmittance at a wavelength of 248 nm wasmerely such that the transmittance after the irradiation for testing was93% of the transmittance immediately after the step (3)

EXAMPLE 18

The same procedure as in Example 17 was followed, except that theabsorption dose of the γ-rays irradiation in step (1) was changed to 10²Gy. An optical fiber thus obtained according to the present inventionwas then irradiated with 10⁸ pulses of KrF excimer laser in the samemanner as in Example 17. A similar reduction in the transmittance tothat realized in Example 17 was observed.

EXAMPLE 19

The same optical fiber (having the same length) as in Examples 17 and18, which is composed of: a silica glass core containing fluorine by 1wt %; and a silica glass clad containing fluorine in an amount of 3 wt%, was irradiated with γ-rays (photon energy of 1.17 Me and 1.33 MeV)the source of which was ⁶⁰Co (step (1)). At this time, the dose ofradiation absorbed by the fiber was 10² Gy. The optical fiber wasimmediately exposed to a hydrogen atmosphere, the temperature of whichwas 80° C. and the pressure of which was 10 atm, for one week (step(2)). At this time, the concentration of hydrogen molecules in theoptical fiber was 7×10¹⁹ molecules/cm³. The fiber was irradiated fromboth ends thereof with 10³ pulses of ArF excimer laser (wavelength: 193nm, 6.4 eV) at 10 mJ/cm²/pulse and 1000 Hz (step (3)).

The optical fiber thus obtained according to the present invention wassubjected to the ultraviolet ray resistance test in such a manner thatthe optical fiber was irradiated from both ends thereof with 10⁴ pulsesof ArF excimer laser. As a result, the deterioration in transmittance ata wavelength of 248 nm was merely such that the transmittance after theirradiation for testing was 92% of the (initial) transmittanceimmediately after the step (3).

EXAMPLE 20

The same procedure as in Example 19 was followed, except thatirradiation of 10⁷ pulses of KrF excimer laser under conditions of 10mJ/cm²/pulse and 1000 Hz was conducted as step (3) in place of the ArFexcimer laser irradiation in Example 19.

The optical fiber thus obtained according to the present invention wasirradiated from both ends thereof with ArF excimer laser as theultraviolet ray resistance test. As a result, the deterioration intransmittance at a wavelength of 248 nm was merely such that thetransmittance after the irradiation for testing was 92% of the (initial)transmittance immediately after the step (3).

EXAMPLE 21

The same optical fiber (having the same length) as in Examples 17 to 20,which is composed of: a silica glass core containing fluorine by 1 wt %;and a silica glass clad containing fluorine in an amount of 3 wt %, wasirradiated with 10⁸ pulses of KrF excimer laser (wavelength: 248 nm, 5eV) at 100 mJ/cm²/pulse and 100 Hz, as step (1). Steps (2) and (3) wereperformed in the same manner as in Example 20. The optical fiber thusobtained according to the present invention was subjected to theultraviolet ray resistance test in such a manner that the optical fiberwas irradiated from both ends thereof with KrF excimer laser. As aresult, the deterioration in transmittance at a wavelength of 248 nm wasmerely such that the transmittance after the irradiation for testing was95% of the (initial) transmittance immediately after the step (3).

EXAMPLE 22

One meter length of an optical fiber which is composed of: a core madeof pure silica glass (SiO₂) containing OH groups in an amount of 1000ppm and chlorine (Cl) in an amount smaller than 1 ppm; and a silicaglass clad containing fluorine in an amount of 3 wt %, and which has alength of 1 m was irradiated with γ-rays (photon energy of 1.17 Me and1.33 MeV) the source of which was ⁶⁰Co in such a manner that the dose ofradiation absorbed by the fiber was 10³ Gy. The optical fiber wasimmediately exposed to a hydrogen atmosphere, the temperature of whichwas 80° C. and the pressure of which was 10 atm, for one week. Theconcentration of hydrogen molecules in the optical fiber was 7×10¹⁹molecules/cm³ after the above step (2) was completed. Step (3) wasperformed in such a manner that the optical fiber was irradiated fromboth ends thereof with 10⁷ pulses of KrF excimer laser (the wavelength:248 nm, 5 eV) at 10 mJ/cm²/pulse and 1000 Hz.

The optical fiber thus obtained was subjected to the ultraviolet rayresistance test in such a manner that the optical fiber was irradiatedfrom both ends thereof with 10⁷ pulses of KrF excimer laser (wavelength:248 nm, 5 eV) at 10 mJ/cm²/pulse and 1000 Hz. As a result, thetransmittance at a wavelength of 248 nm was not substantially changed.However, when the test condition was changed to a 10⁸ pulse irradiation,the transmittance at a wavelength of 248 nm was reduced to about 74% ofthe transmittance after step (3).

EXAMPLE 23

The optical fiber according to the present invention which is obtainedin Example 16 was again irradiated with γ-rays in such a manner that theabsorption dose was 10⁶ Gy. As a result, increase in the transmissionloss of 100 dB/km or greater was observed with the foregoing opticalfiber at a wavelength of 500 nm.

A high-purity silica glass article is subjected to step (1) in which thehigh-purity silica glass article is irradiated with electromagneticwaves having energy with which defects can be generated, that is, photonenergy not smaller than 3.5 eV so that defects are generated in theglass, and subjected to step (2) in which the high-purity silica glassarticle is immersed in an atmosphere composed of a hydrogen gas. Thus,the defects are allowed to disappear so that substantial increase in thelight absorption in the ultraviolet ray region having a wavelength offrom 160 nm to 300 nm occurring due to ultraviolet ray irradiation isprevented. Since irradiation with γ-rays is able to simultaneouslyprocess a large quantity of fibers and a small dose of 10⁴ Gy orsmaller, preferably 10³ Gy is sufficiently large to obtain asatisfactory effect of preventing deterioration due to ultraviolet rayirradiation, a problem of deterioration in the coating for the fiber canbe prevented.

Since step (3) in which electromagnetic waves are applied enableshydrogen in the glass article to be fixed, an article, such as theoptical fiber, having a small diameter with respect to the diffusingspeed of hydrogen is enabled to maintain ultraviolet ray resistance fora long time.

When step (4) of removing hydrogen is performed to follow step (3),surplus hydrogen in the glass can be removed. Thus, light absorptionperformed by the hydrogen molecules can be reduced and thecharacteristic can be stabilized.

Moreover, an advantage can be obtained in that the silica glass articlehas excellent light transmission in the vacuum ultraviolet ray region ascompared in the atmosphere. Since the silica glass article hasflexibility, the silica glass article according to the present inventioncan significantly advantageously be employed in an apparatus using anultraviolet ray source, such as excimer laser beams, a deuterium lamp ora halogen lamp, in particular as a light transmission medium for amachining apparatus, for example, a laser machining apparatus, aphotolithography apparatus, a fiber hardening ray source, an adhesioncuring ray source, a variety of microelement machining apparatuses andan SR (synchrotron) light generating source.

For the confirmation of the effect of the present invention, thefollowing examples and comparative examples were further conducted.Irradiation conditions in the ultraviolet ray resistance tests performedto evaluate each optical fiber according to each example and comparativeexample were such that irradiation with a KrF excimer laser of awavelength 248 nm, 10 mJ/cm²/pulse, and 10⁸ pulses were applied to bothends of the optical fiber.

EXAMPLE 24

As shown in FIG. 10, a mono-core optical fiber of 1,000 meter lengthcomposed of silica glass core 1 containing Cl in an amount lower than 1ppm and fluorine (hereinafter referred to as (F)) in an amount of 1 wt%, and silica glass clad 2 containing F in an amount of 3 wt % wasimmersed in a hydrogen atmosphere, the pressure of which was 10 atm, at80° C. for one week (step a), taken out from the hydrogen atmosphere,and then the optical fiber was cut to a length of 1 m and irradiatedfrom both ends thereof with 10⁷ pulses of a KrF excimer laser having awavelength of 248 nm at 10 mJ/cm²/pulse as shown in FIG. 9 (step b). Atthis time, the concentration of hydrogen molecules in the optical fiberwas 7×10¹⁹ molecules/cm³.

The ultraviolet ray resistance test was performed in such a manner thatthe above-obtained optical fiber having a length of 1 m was irradiatedwith 10⁶ pulses of a KrF excimer laser. The transmittance at awavelength of 248 nm after irradiation was merely deteriorated to about96% of the transmittance at initial stage (the transmittance immediatelyafter the final step of the present invention and before the irradiationfor testing). The optical fiber showed sufficient ultraviolet rayresistance characteristics.

EXAMPLE 25

A mono-core optical fiber of one meter length composed of silica glasscore containing Cl in an amount lower than 1 ppm and fluorine (F) in anamount of 1 wt %, and silica glass clad containing fluorine (F) in anamount of 3 wt % was allowed to stand in a hydrogen atmosphere, thepressure of which was 10 atm, at 80° C. for one week, taken out from thehydrogen atmosphere, and irradiated from both ends thereof with 10⁷pulses of a KrF excimer laser having a wavelength of 248 nm at 10mJ/cm²/pulse as shown in FIG. 9. At this time, the concentration ofhydrogen molecules in the optical fiber was 1×10¹⁹ molecules/cm³. Theoptical fiber obtained was allowed to stand in an atmosphere at 40° C.for three weeks so that removal of hydrogen was performed. As a result,the concentration of hydrogen molecules in the optical fiber became lessthan 1×10¹⁶ molecules/cm³.

The thus-obtained optical fiber according to the present invention (1 mlength) was subjected to the ultraviolet ray resistance test in the samemanner as in Example 24. As a result, the deterioration in transmittanceat a wavelength of 248 nm was merely such that the transmittance afterthe irradiation for testing was about 93% of the transmittance atinitial stage (the transmittance immediately after the final step of thepresent invention and before the irradiation for testing). The opticalfiber exhibited sufficient ultraviolet ray resistance characteristics.As a result of hydrogen removal treatment, an optical fiber having nofluctuation in refractive index with the lapse of time could beobtained.

EXAMPLE 26

The same mono-core optical fibers used in Example 25 were bundled toform a bundle fiber (1 m length) as shown in FIG. 11. This bundle fiberwas subjected to processes of each step according to the presentinvention in the same manner as in Example 25 to obtain the bundle fiberof the present invention. The thus-obtained bundle fiber (1 m) wassubjected to the ultraviolet ray resistance test in the same manner asdescribed above and evaluated. The same advantageous result that thetransmittance at a wavelength of 248 nm after irradiation was merelydeteriorated to about 96% of the transmittance at initial stage could beobtained.

EXAMPLE 27

An optical fiber according to the present invention was obtainedaccording to the same procedure as in Example 24 except that theirradiation with a KrF excimer laser was performed only from one side ofthe optical fiber.

The thus-obtained optical fiber (1 m length) was subjected to theultraviolet ray resistance test in the same manner as in Example 24 andevaluated. The transmittance at a wavelength of 248 nm after irradiationwas merely deteriorated to about 93% of the transmittance at initialstage, which was the same advantageous result as in Example 24.

Comparative Example 3

A mono-core optical fiber (1,000 meter length) composed of silica glasscore containing Cl in an amount lower than 1 ppm and fluorine (F) in anamount of 1 wt %, and silica glass clad containing F in an amount of 3wt % was allowed to stand in a hydrogen atmosphere at 80° C. for oneweek, the pressure of which was 10 atm, taken out from the hydrogenatmosphere, and then the optical fiber was cut to a length of 1 m andirradiated by a deuterium lamp from both ends thereof for 24 hours asshown in FIG. 12. At this time, the concentration of hydrogen moleculesin the optical fiber was 7×10¹⁹ molecules/cm³. Irradiation conditionswere such that the output from the lamp was 150 W and the distance fromthe lamp to the fiber was 15 cm. The irradiation was conducted from bothends of the optical fiber.

The thus-obtained optical fiber for comparison (1 m length) wassubjected to the ultraviolet ray resistance test in the same manner asin Example 24. As a result, the transmittance at a wavelength of 248 nmwas remarkably deteriorated such that the transmittance after theirradiation for testing was about 30% of the transmittance at initialstage (the transmittance immediately after the final step and before theirradiation for testing).

Comparative Example 4

A mono-core optical fiber of one meter length composed of silica glasscore containing Cl in an amount lower than 1 ppm and fluorine (F) in anamount of 1 wt %, and silica glass clad containing F in an amount of 3wt % was allowed to stand in a hydrogen atmosphere at 80° C. for oneweek, the pressure of which was 10 atm, taken out from the hydrogenatmosphere, and then irradiated from both ends thereof with 10⁵ pulsesof a KrF excimer laser having a wavelength of 248 nm at 10 mJ/cm²/pulseas shown in FIG. 9. At this time, the concentration of hydrogenmolecules in the optical fiber was 1×10¹⁹ molecules/cm³. The opticalfiber obtained was allowed to stand in an atmosphere at 40° C. for threeweeks so that removal of hydrogen was performed. As a result, theconcentration of hydrogen molecules in the optical fiber became lessthan 1×10¹⁶ molecules/cm³.

The thus-obtained optical fiber for comparison (1 m length) wassubjected to the ultraviolet ray resistance test in the same manner asin Example 24. As a result, the transmittance at a wavelength of 248 nmwas remarkably deteriorated such that the transmittance after theirradiation for testing was about 30% of the transmittance at initialstage (the transmittance immediately after the final step and before theirradiation for testing).

As explained above, the starting material silica glass article issubjected to the step in which hydrogen molecules are introduced intothe silica glass article, and subjected to the step in which the silicaglass article is irradiated with an excimer laser having a wavelength of248 nm at an appropriate quantity while the hydrogen molecules remainingin the optical fiber is not lower than 1×10¹⁶ molecules/cm³ to therebyforcibly convert defect precursors in the glass to defects to form astable bonding between the defects and the hydrogen molecules. Thus,hydrogen can be fixed. The substantial increase in the light absorptionin the ultraviolet ray region having a wavelength of from 160 nm to 300nm, the increase occurring due to ultraviolet ray irradiation, isprevented. In particular, due to the fixation of hydrogen, an article,such as the optical fiber, having a small diameter with respect to thediffusion rate of hydrogen is enabled to maintain ultraviolet rayresistance for a long period of time. In addition, preliminaryirradiation of electromagnetic waves is not required for convertingprecursors to defects and the conversion and the hydrogen fixation canbe performed concurrently, which results in the improvement ofproductivity.

Further, by the addition of the step of removing hydrogen, surplushydrogen not fixed in the glass can be removed. Thus, light absorptionby the hydrogen molecules can be reduced, the characteristics of thearticle can be stabilized, fluctuation in refractive index with thelapse of time can be prevented and constant luminance can be obtained,and the use of a light in near infrared region becomes feasible becausethe absorption of the light by the hydrogen molecules does not occur.

Conventional fibers for use in the ultraviolet region are easily liableto be deteriorated due to ultraviolet irradiation thereforeunpracticable. According to the process of the present invention, fiberscan be used accompanied with no problems in the wavelength region offrom 300 to 200 nm, further, even in a vacuum ultraviolet ray regionwithout vacuuming.

Moreover, an advance can be obtained in that the silica glass articlehas excellent light transmission in the vacuum ultraviolet ray region ascompared in the atmosphere. Since the silica glass article hasflexibility, the silica glass article according to the present inventioncan significantly advantageously be employed in an apparatus using anultraviolet ray source, such as excimer laser beams, a deuterium lamp ora halogen lamp, in particular, as a light transmission medium for amachining apparatus, such as a laser machining apparatus or aphotolithography apparatus, and a light transmission medium for a fiberhardening ray source, an adhesion curing ray source, a variety ofmicroelement machining apparatuses or an SR (synchrotron radiation)light generating source.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A silica glass article which is manufactured by aprocess comprising the steps of: (1) irradiating a silica glass articleserving as a raw material with electromagnetic waves to generate defectstherein; (2) immersing the thus irradiated silica glass article in anatmosphere comprising a hydrogen gas, thereby providing the resultingsilica glass article with a characteristic that is effective forpreventing it substantially from increasing its absorption within anultraviolet region due to ultraviolet ray irradiation; and (3)irradiating again the silica glass article that has been subjected tothe step, (2), with electromagnetic waves while hydrogen moleculesremain therein.
 2. The silica glass article according to claim 1,wherein the process further comprises, after the step (3), a step of (4)reducing the hydrogen molecules that are remaining in the silica glassarticle to not higher than 1×10¹⁶ molecules/cm³ by at least one ofallowing it to stand in the atmosphere and heating at 80° C. or lower.3. A silica glass article which is manufactured by a process comprisingthe steps of: (a) introducing hydrogen molecules to the silica glassarticle to have a hydrogen concentration of not lower than 1×10¹⁶molecules/cm³, and (b) irradiating the hydrogen-introduced silica glassarticle with an excimer laser or not more than 100 Gy of γ-rays whilethe concentration of the hydrogen molecules present in the silica glassarticle is not lower than 1×10¹⁶ molecules/cm³, thereby providing theresulting silica glass article with a characteristic that is effectivefor preventing it substantially from increasing its absorption within anultraviolet region due to ultraviolet ray irradiation such that thesilica glass article has a transmittance after 10⁸ irradiation pulses ofnot less than 92% of the transmittance immediately after production. 4.The silica glass article according to claim 3, wherein the processfurther comprises: c) reducing the concentration of the hydrogenmolecules which are present in the silica glass article to not higherthan 1×10¹⁶ molecules/cm³ by at least one of allowing it to stand in theatmosphere and heating at a temperature not higher than 80° C.
 5. Thesilica glass article having defects generated when its precursor isirradiated with a photon energy of not less than 3.5 eV, hydrogen atomsbeing bonded to the defects, wherein the silica glass article has acharacteristic that it is substantially prevented from increasing itsabsorption within an ultraviolet region after irradiation by a deuteriumlamp for 48 hours.